Let me start this out by saying that I am a mechanical engineer working in the Aerospace industry, specifically on HVAC. Between myself and an entire experienced engineering department at work, none of us can figure this one out. So here's the summary:

Bought a 2002 Audi TT Quattro Coupe ALMS Edition with 90k miles in June of 2015. Was having an A/C issue where the system wouldn't cool at idle or low RPM's on hot days (above 80 degrees), only at higher RPM's. Also would get a weird "groaning" sound from the compressor at times, which would go away if I revved the engine. The system would cool just fine at highway speeds and the condenser fans worked just fine. Reclaimed the refrigerant using a cart from work, held a vacuum, then recharged to the Audi-specified charge. Didn't resolve anything. The strange part is that I could drive the car home from work (about a 20 minute drive) and park in the driveway for 15 minutes with the car idling and it would cool just fine with no issues. I should also note that I never have any engine over-heating issues.

Recently I hooked up a set of gauges and recorded the pressures on a hot day with the A/C on full blast as low as it would go and with the windows open. At first the pressures seems stable and held at ~45 PSIG low side, ~260 PSIG high side, the compressor was engaged, both condenser fans were engaged, and cold air was blowing out of the vents. Then I can hear a slight change from the engine (hard to describe, but the compressor and condenser fans are still engaged) and the high side pressure rises rapidly until it hits ~475 PSIG (presumable the high pressure limit) and disengages the compressor (low side remains at ~45 PSIG until the compressor disengages). This whole time, the condenser fans are still running, and they continue to run until the high side pressure gets back down to ~350 PSIG and the compressor reengages. High side pressure will rise again until it hits the high pressure limit again and disengages the compressor. This cycle will occur a couple times then the compressor will stay engaged, but the high side pressure will hold between 350 and 400 PSIG.

I don't believe there is an issue with the compressor, since it actually works at times. Same with the receiver dryer and the expansion valve. I have already replaced the high pressure switch. So my last guesses are either the radiator fan switch (https://www.ecstuning.com/b-genuine-...ck/1h0959481b/) or the auxiliary fan control unit (https://www.ecstuning.com/b-genuine-...it/1j0919506k/). Any other thoughts or anything I am missing?

Also, I recorded a video of when I was checking things with the gauges. Around the 2:10 mark you will see when the high side pressure rises. I did notice after re-watching the video that after the compressor cycles the first time that the fans can be heard going into "high speed" mode. See video here:
Sorry for the long post, but thanks in advanced!

- Dalton

 

Hi Dalton. Living in Tecumseh you should know something about compressors!

Having come across your post whilst looking for a solution to my problem programming an A4 B5 CC unit, I joined the forum to reply.

I know you measured the charge, but it sounds to me that the system is overcharged.

Provided the low side pressure stays the same and the condenser fans are running there is really no other explanation than that the condenser is too full of liquid and cannot adequately condense the HP gas.

EDIT: You didn't by chance charge Kgs instead of Lbs?

 

And being from France, I'm wondering if you have any connections to Zodiac Aerospace. Just curious

Yes, going off my experience with compressors, it sounds like it may be over charged. But back in 2015 when we reclaimed the system and charged it again, we charged to the value specified on the label under the hood, which was 750 +/- 25 grams, which equates to 1.65 +/- .11 pounds. Even at this charge, I was having the same issues.

I did add slightly more refrigerant last year on the off chance that there may have been a leak, but it had no affect. So I actually reclaimed some refrigerant today and I am going to see if things improve any.

On a related topic, does anyone know of any specific measuring blocks I could monitor with VCDS to troubleshoot my HVAC issues? All I can find is compressor on/off status, A/C high or low status, and a few temperature readouts. Anything that is related to the auxiliary fan, fan switch, evaporator temp, compressor current or voltage, etc? I'm having trouble finding this info anywhere online.

Thanks,
Dalton

 

I misread your post, and took it you had just recharged your system, but it is still possible that it has been overcharged fror two years. A huge proportion of systems of all sizes that I have worked with were overcharged. It seems to be the standard solution to all problems to "add a bit of gas".

Did you recharge the recovered refrigerant or use new? and how did you weigh the charge?

Re. measuring blocks, I can't help with specifics, they seem to be a well-kept secret, only known to those who charge large sums for their witchcraft.

However, it's easy to see/hear that the fan operates, the suction pressure relates directly to the evap temp, and the compressor is engine driven, so only the clutch draws any current.

 

Since my previous post, I recovered some refrigerant, assuming that I may simply be overcharged. It seem like this has mostly solved my issues, although the cooling performance still leave something to be desired.

And as for the question about how I charged the system in the past, I used a Robinair automotive cart, which is what we use for all our HVAC development and testing at work. If I remember correctly, we recovered a little less than the recommended, pulled a vacuum, then recharged the recommended amount. It was not virgin refrigerant, but I have no reason to believe there was anything wrong with it.

Thank you everyone for your assistance! I will continue to monitor the car and see how it goes.

 

If cooling is poor it is possible the refrigerant is contaminated with air and/or moisture.

As the cost of a new charge is not really significant I'm surprised you recharged with the old refrigerant, but then, I last worked in the days of R12 and R22, when refirigerant was as cheap as chips and no-one considered re-using it.